Modern medical care often involves the use of medical pump devices to deliver substances, such as fluids and/or fluid medicine to patients. Medical pumps permit the controlled delivery of substances to a patient, and such pumps have largely replaced gravity flow systems, primarily due to the pump's much greater accuracy in delivery rates and dosages, and due to the possibility for flexible yet controlled delivery schedules.
A typical positive displacement pump system includes a pump device driver and a disposable fluid or pumping chamber, defined in various forms including but not limited to a cassette, syringe barrel or section of tubing. A disposable cassette, which is adapted to be used only for a single patient and for one fluid delivery round, is typically a small plastic unit having an inlet and an outlet respectively connected through flexible tubing to the fluid supply container and to the patient receiving the fluid. The cassette includes a pumping chamber, with the flow of fluid through the chamber being controlled by a plunger or pumping element activated in a controlled manner by the device driver.
For example, the cassette chamber may have one wall or wall portion formed by a flexible, resilient diaphragm or membrane that is reciprocated by the plunger and the driver to cause fluid to flow. The pump driver device includes the plunger or pumping element for controlling the flow of fluid into and out of the pumping chamber in the cassette, and it also includes control mechanisms to assure that the fluid is delivered to the patient at a pre-set rate, in a pre-determined manner, and only for a particular pre-selected time or total dosage.
The fluid enters the cassette through an inlet and is forced through an outlet under pressure. The fluid is delivered to the outlet when the pump plunger forces the membrane into the pumping chamber to displace the fluid. During the intake stroke the pump plunger draws back, the membrane covering the pumping chamber pulls back from its prior fully displaced configuration, and the fluid is then drawn through the open inlet and into the pumping chamber. In a pumping stroke, the pump plunger forces the membrane back into the pumping chamber to pressurize and force the fluid contained therein through the outlet. Thus, the fluid flows from the cassette in a series of spaced-apart pulses rather than in a continuous flow.
One of the requirements for a medical pump is that it is able to deliver precise volumes at precise delivery rates. Conventional pumps, in general, rely on nominal or empirical data to estimate the delivery volumes and delivery rates, and do not provide mechanisms for adjusting an actual delivery due to variations from this nominal or empirical data. This lack of adjustment during an actual delivery limits the accuracy and/or flow continuity of these pumps.
In addition, medical pumps are operated at low flow rates, such as below 1 mL/hr or less, the determination of when the medical pump is actually delivering a substance to a patient can be difficult. It has been found that sensed data can provide false indications that actual delivery of the substance, such as the flow of a fluid, is occurring. In fact, it has been determined that sensed data indicating that delivery of the substance has begun can actually be attributed to leakage or some other reason, as suggested by the sensed data, such as pressure, instead of the delivery actually beginning. Other potential difficulties occur when attempting to use traditional medical pumps at low flow rates, without using specialty items such as specialty neonatal cassettes. In particular, mechanical friction and/or electrical noise can also trigger false data indicating that the delivery has actually begun, inducing periods of no flow. This friction and/or noise can be attributed to many things, including but not limited to the cassette diaphragm, the plunger tip finish, and/or the plunger body O-rings to internal bearing pressure/force sensor flex bias.
Thus, it is a principal object of this invention to provide a medical pump and a method of operating a medical pump to overcome these deficiencies and accurately deliver a substance to a patient, such as an infant, in smaller increments for low flow rates in a more continuous manner (known as Low Flow Continuity). In general, Low Flow Continuity is defined as the ability of a pump to deliver at rates of 1 ml/hr to 0.1 ml/hr or less with periods of “no-flow” not exceeding 20 seconds and bolus volumes not exceeding 2 micro-liters. To meet the highest Emergency Care Research Institute (ECRI) industry standards for Low Flow Continuity and achieve an “Excellent” ECRI rating, the pump must at least deliver fluid in increments no greater than two micro-liters at a flow rate of 0.1 milliliter per hour with a maximum “no-flow” period of 20 seconds.
The present invention is provided to solve the problems discussed above and other problems, and to provide advantages and aspects not provided by prior medical pumps. A full discussion of the features and advantages of the present invention is deferred to the following detailed description, which proceeds with reference to the accompanying drawings.